From what I can tell, and I've only been reading the docs for a few days now. Everything is considered an actor.... Users, comments, posts, communities and even the instance itself. When you create a new post to a community you're essentially replying to that community's actor. If that community is not on your instance then your instance needs to post that reply via an ActivityPub service call. Now once the target instance receives that post request it adds it to it's database but then that instance needs to inform all of the other instances that something has been added to that community. So it'll queue up a service call to every other instances that has federated with that community's instance.

So in the end if you're a user on instance A but trying to create a post to a community that belongs to instance B, A has to tell B about the post. Next B then, since it's the owner of that community, has to inform instance C,D,E, etc... that there's now a new reply to that community.

Again I might still be missing something as I've only been reading these for about a day or so. But if this is correct then each action anyone takes produces a lot of network traffic to update every other instance in the fediverse.